In the proposed experiments, rabbits will be used to test the hypothesis that FSH is the principal trigger of a wave of follicle growth in the postovulatory ovary. The project is divided into three general parts: 1) The exact time of onset, duration, and termination of the postovulatory release of FSH in the rabbit will be determined. 2) The pattern of follicular growth and regression in pregnant and nonpregnant ovaries will be determined. 3) Once changes in these parameters have been defined, the resulting information will be used in formulating experiments to determine the effects on follicular growth and gonadotropin release of manipulating FSH levels and the levels of postovulatory steroids. The experiments will involve artificially raising blood levels or blocking FSH release with the effects on follicle growth noted. Similarly, steroid concentration in the blood will be manipulated using a specific antibody and by injection of exogenous steroids. The results of these experiments will be used to accept or reject the hypothesis that FSH initiates or accelerates the rate of postovulatory follicle growth. Experiments will also investigate the possibility that the postovulatory follicular steroid secretion may alter LH secretion such that only FSH is released in the second gonadotropin surge which follows ovulation. The rabbit seems to be an excellent animal model system for the investigation of the factors which control postovulatory follicle growth. The does will be used through the first six days of pregnancy or pseudopregnancy because this is a period when follicle growth must occur to replace those large follicles lost to ovulation. The studies are of great importance to the understanding of the factors controlling the development of follicles. Clearly, since the growth of ovarian follicles is an integral part of the mammalian reproductive process, interruption or alteration in follicle growth whould be of potential value as a contraceptive.